The present invention is directed to an electrochemical sensor that can be employed for the determination of the concentrations of gases such as, O.sub.2 or CO.sub.2.
U.S. Pat. No. 5,376,255 discloses a gas sensor wherein an electrochemical sensor is fashioned on a planar substrate of semiconductor material such as silicon. This sensor comprises a pH-sensitive electrode such as IrO.sub.2, or a pH-sensitive ISFET (ion-selective field effect transistor). Independently of the sensor, which is matched to the gas (O.sub.2, CO.sub.2) to be defined, a reference electrode preferably composed of a precious metal is present in such a sensor. For example, this precious metal is platinum. In potentiometric sensors, however, silver, silver chloride electrodes are present.
The sensor and reference electrode are covered with an electrolyte layer. This electrolyte layer is surrounded on all sides by an annular structure of polyimide or some other polymer such as, for example, polybenzoxazole. A hydrophobic layer is applied over the electrolyte layer and laterally relative thereto. The hydrophobic layer being in turn surrounded by a second or outer polymer structure. The outer annular polymer structure is higher than the inner polymer structure that encloses the electrolyte layer.
The problem arises with this arrangement is that there is an inadequate adhesion between the hydrophobic layer, which is usually polysiloxane, and the polyimide structure. A crack is therefore formed between the hydrophobic layer and the framing. Due to the slight thickness of the hydrophobic layer, approximately 20 .mu.m through 30 .mu.m, this crack formation results therein that an analysis fluid (electrolyte solution or blood) to be examined can enter the crack and short-circuits the electrode of precious metal with a cooperating electrode of the sensor or electrically conductive regions of the sensor base structure. The fluid penetration can also result in corrosion of one or both of the electrodes.
Therefore, there is a need for an improved electrochemical sensor with a structure that is not prone to cracking or to fluid leakage which can result in a short circuiting or corrosion of the sensor.